Low molecular weight chitosan, product of γ-ray degradation, was modified by introducing glycidyl trimethyl ammonium chloride group and its antibacterial property was tested against different bacteria. The structure of chitosan and chitosan quaternary ammonium salt are characterized by FT-IR and ,13,C NMR. The results showed that the degree of substitution of the quaternary ammonium salt of chitosan with γ-ray irradiation was higher than original and its antibacterial effect to Escherichia coli and Staphylococcus aureate was also improved obviously, and molecular structure of chitosan wasn’t destroyed by γ-ray.